In hydraulics, power is transmitted by means of pressure and volume flow. Normally, the volume flow is generated by a pump, and the resistance caused by the system increases the pressure. Normally, the aim is to move actuators, such as cylinders and motors, by means of the volume flow and the pressure. Because the actuators must normally be moved in both directions, either back and forth or changing the direction of circulation, and possibly even be stopped, valves are needed for controlling the volume flow. When the actuators are to be moved precisely, for example at a specific rate, accurate valves are needed to control the volume flow. Conventionally, such valves are so-called proportional valves. Proportional valves have been used already for decades, and there are several different valves functioning on the same basic principle. FIG. 1 shows a way of coupling a proportional valve to a hydraulic actuator, and FIG. 2 shows the characteristic curve of a conventional four-way proportional valve. In practice, a certain control signal u is used to achieve a position of the slide of the proportional valve to provide simultaneous openings of the proportional valve of given magnitude (i.e. simultaneous control of two control edges), for example from port P to port A and from port B to port T (FIG. 1).
Better controllability is achieved if more than one proportional valve is used for controlling the actuator, because all the control edges can be controlled independently, irrespective of the each other. In FIG. 3, such a control with four control edges is implemented by 2/2 proportional valves. By means of the system, better efficiency and controllability are achieved, but on the other hand, the implementation is expensive due to the high prices of the single valves.
A digital hydraulic valve system is a kind of a proportionally functioning directional valve. The valve system consists of simple and robust on/off valves coupled in parallel. The valves are arranged in such a way that by their different opening combinations it is possible to achieve a gradual control response at each control edge. This technique is also known as Pulse Code Modulation, or PCM. Document U.S. Pat. No. 2,999,482 is mentioned as an example. FIG. 4 shows one control edge of a digital hydraulic valve system, or Digital Flow Control Unit (DFCU), and its control response, when the volume flow rates of the valves are selected so that the following valve is always twice as large as the preceding one. In the control response, the flow rate Q is shown as a function of the control input.
The digital hydraulic valve system normally consists of either two or four separate DFCU's. A single DFCU can be made in several ways, of which the binary, Fibonacci and Pulse Number Modulation (PNM) based solutions are the most examined ones. The basic principle of the control of the digital hydraulic valve system is discussed in WO 02/086327 A1, which corresponds to patent application FI 20010827 A.
Table A shows, in a general case, the ratio between the opening combinations and the total opening of a valve series in one DFCU (Qj: the flow rate through a valve j). It can be seen in the table that 16 different flow rate values can be obtained with four valves. In a corresponding manner, 32 and 64 flow rate values would be obtained with five and six valves, respectively.
FIG. 5 shows a digital hydraulic valve system implemented with four separately adjustable control edges. Each valve series typically comprises 4 to 6 valves coupled in parallel, wherein the whole valve system comprises 16 to 24 valves.
TABLE ADFCUstateValve 1Valve 2Valve 3Valve 4Flow rate00000011000Q120100Q231100Q1 + Q240010Q351010Q1 + Q360110Q2 + Q371110Q1 + Q2 + Q380001Q491001Q1 + Q4100101Q2 + Q4111101Q1 + Q2 + Q4120011Q3 + Q4131011Q1 + Q3 + Q4140111Q2 + Q3 + Q4151111Q1 + Q2 + Q3 + Q4
In proportional valves of prior art, fault tolerance is primarily limited to the robust structure and fail-safe properties of the valve. In practice, if there is a fault in the valve, the input current to the valve is switched off and so-called centering springs drive the slide either to the central position or to another “safe” position. In some feedback systems, in case of a fault it is possible to try to increase for example the input current to achieve a desired position of the stem. In some valves, a fault condition is indicated to the operator either by means of light signals (e.g. LED) or, for example, a CAN message (Controller Area Network). Even in these cases it is still not possible to speak about actual fault tolerance. In some critical uses, such as ships and aircrafts, tandem systems may be used, with a duplicate component for each component. These duplicate components can be used in case of a fault, and the faulty part is disconnected from the system. In some cases, only those parts which are susceptible to failure are duplicated.
In the system of four proportional valves shown in FIG. 3, some kinds of operations can be performed even under a failure. With the system, it is possible, for example, to move an actuator in only one direction or, in some cases, to stop an actuator moved by a valve that has been jammed open. However, in practice, the system is unserviceable in the case of one faulty valve.
The valve system of FIG. 5, comprising four separate valve series, has been discussed in the conference publication ‘Digital hydraulic tracking control of mobile machine joint actuator mockup’; Linjama M., Vilenius M., The Ninth International Conference on Fluid Power, SICFP'05, Jun. 1-3, 2005 Linköping, Sweden. The system of FIG. 5 has also been introduced in the article ‘Improved digital hydraulic tracking control of water hydraulic motor drive’; Linjama M., Vilenius M., International Journal of Fluid Power, Volume 6, Number 1, March 2005.
In the system according to the article, the control and, for example, solely the extend stroke of the actuator is implemented by using even three or all the four valve series, whose all opening combinations are worked out, and the suitable one is selected by optimation. However, the article does not point out a fault situation, its detection, nor a measure to compensate for a failure of the valves and the effect of the fault situation on the control.